Re: impact of 4k sector size on the IO & FS stack

Ric Wheeler wrote:

Alan Cox wrote:

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First generation of 1K sector drives will continue to use the same
512-byte ATA sector size you are familiar with.  A single 512-byte
write will cause the drive to perform a read-modify-write cycle. 
This configuration is physical 1K sector, logical 512b sector.

The problem case is "read-modify-screwup"

At that point we've trashed the block we were writing (a well studied
recovery case), and we've blasted some previously sane, totally
unrelated sector of data out of existance. Thats why we need to know
ideally if they are doing the write to a different physical block when
they do this, so that we don't lose the old data. My guess is they won't
as it'll be hard.

I think that the firmware would have to do this in the drive's write
cache and would always write the modified data back to the same physical
sector (unless a media error forces a sector remap).

If firmware modifies the 7 512 byte sectors that it read to do the 1 512
byte sector write, then we certainly would see what you describe happen.

In general, it would seem to be a bad idea to do allocate a different
physical sector to underpin this king of read-modify-write since that
would kill contiguous layout of files, etc.

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A future configuration will change the logical ATA interface away
from 512-byte sectors to 1K or 4K.  Here, it is impossible to read a
quantity smaller than 1K or 4K, whatever the sector size is.

That one I'm not worried about - other than "guess how Redmond decide to
make partition tables work" that one is mostly easy (be fun to see how
many controllers simply can't cope with the command formats)

This will be interesting to find out. I will be sharing a panel with
some BIOS & MS people, so I will update all on what I hear,

Ric,
Just to add a SCSI perspective, it looks like 4 KB sectored
disks will be almost exclusively ATA devices. It is being
done to improve capacity at the expensive of performance.
[SCSI/FC/SAS disks typically trade off capacity for better
performance.]

Support for disks with smaller logical block size than
physical block size has already been added to SBC-3. The
overview of this document gives a rationale:
www.t10.org/ftp/t10/document.06/06-034r5.pdf

SAT is now a standard and an agenda item for SAT-2 is
to wire ATA8-ACS's large sector size support to the
additions to SBC-3 mentioned above.


I'm not sure how this stuff plays with end to end data
protection :-)
Most SCSI disks currently allow formatting sizes of 512
up to 528 bytes per logical block.

Doug Gilbert